#include <iostream>
#include <fstream>

int main()
{       
   std::ifstream inf("in.txt");
   std::ofstream outf("out.txt");
   double firstx, firsty;
   inf >> firstx >> firsty;
   double x, y;
   x = firstx;
   y = firsty;
   double x1, y1, area = 0;

   while(inf >> x1 >> y1)
   {
      area += (x - x1) * (y + y1);   
	  x = x1;
	  y = y1;
   }
   area  += (x - firstx) * (y + firsty);
   area /= 2;

   outf << std::fixed << abs(area) << std::endl;

   inf.close();
   outf.close();
   return 0;
}